In the well known electro-smelting process of ilmenite, metallic iron and a slag enriched in TiO.sub.2 is produced. Such slags are especially appreciated by the TiO.sub.2 pigment producers using the sulfate process route. A typical analysis of these slags is:
______________________________________ TiO.sub.2 71.0% FeO 11.8% SiO.sub.2 5.2% Al.sub.2 O.sub.3 5.5% CaO 0.8% MgO 5.1% V.sub.2 O.sub.5 0.58% Cr.sub.2 O.sub.3 0.18% MnO 0.24% ______________________________________
However, because of the impurities contained in these slags, these slags are not generally accepted by the chloride process TiO.sub.2 pigment producers. To be suitable for the latter process, a slag must have essentially a low CaO and a low MgO content and secondarily a low Al.sub.2 O.sub.3 and SiO.sub.2 content. Typically, these constituents are found in the following ranges when suitable as a starting material in the chloride process:
______________________________________ CaO from 0.0 to 0.2 in weight percent MgO from 0.0 to 1.0 in weight percent Al.sub.2 O.sub.3 from 0.0 to 1.0 in weight percent SiO.sub.2 from 0.0 to 1.5 in weight percent ______________________________________
The undesirable impurities CaO and MgO are unwanted because these generate high boiling point chlorides in the process. These chlorides tend to accumulate in the fluidized bed used in the chloride process and, under operating conditions, remain in a molten form causing lower chlorination rate, bed sticking and defluidization.
The two other impurities mentioned above, namely Al.sub.2 O.sub.3 and SiO.sub.2, are essentially high chlorine gas consumers which increase the cost for producing TiO.sub.2 pigment and complicate the purification of TiCl.sub.4 as well as aggravate scrubbing problems of the TiCl.sub.4 distillation residues.
In other TiO.sub.2 source materials, including slags, depending on the origin of the ore, MnO could also be present. Manganese oxide is easier to distill from the reaction mixture than MgCl.sub.2 and CaCl.sub.2, however, it is also undesirable in large amounts.
The chloride process to produce titanium is based on the intermediate formation of titanium tetrachloride according to the following equation: EQU TiO.sub.2 +2C+2Cl.sub.2 .fwdarw.TiCl.sub.4 +2CO (or CO.sub.2)
The preferred raw material is natural or artificial rutile, the latter being manufactured from ilmenite by various processes.
Ilmenite is also widely used as a raw material for TiO.sub.2 pigment production. In the chlorination process, impurities associated with ilmenite, principally iron, cause the formation of large amounts of iron chlorides as either ferrous or ferric chlorides. The chlorides must be disposed of or processed for chlorine recycling.
Several processes for chlorination of high alkali-earth slags have been tried or proposed. The only one which heretofore has been effectively used involved the blending of a slag, such as Sorelslag, either with rutile or ilmenite with low CaO and MgO content. Such practice implies a source of these feeds and does not solve the problems mentioned above.
Another approach for avoiding the problem plagued use of slag consists of making briquets of slag and carbonaceous material. This is an impractical process because it necessitates batch operation. Still other processes have been operated at higher temperature in a fluid bed eventually with an excess of chlorine, such as disclosed in U.S. Pat. No. 2,974,009, and in presence of excess of carbon which must be continuously discharged, such as disclosed in U.S. Pat. No. 3,074,777, as well as where the starting material is washed before recycling including any unreacted material associated therewith (described in U.S. Pat. No. 2,701,180). Still another proposal has been to use a new reactor to produce flash chlorination of these slags such as disclosed in U.S. Pat. No. 3,787,556.
In absence of an oxygen acceptor, the chlorination rate of titanium dioxide is very low even at tempteratures as high as 1000.degree. C. In presence of an oxygen acceptor, e.g., carbon, the reaction proceeds already at temperatures as low as 500.degree. C. Two different mechanisms are involved in the temperature ranges of 500.degree.-800.degree. C. and above 900.degree. C. as further amplified. At low temperatures, good contact between TiO.sub.2 and carbon is necessary to get the reaction started while at high temperature a close contact is not imperative. Based on these facts, the briquetting process and also pelletizing and shaft furnace chlorination with continuous discharge of the unreacted material, such as disclosed in U.S. Pat. No. 3,359,065, does permit a low temperature reaction.
The reducing agent, so-called oxygen acceptor, generally employed in actual chlorinators is petroleum coke which is also a high cost material. In the present invention the carbon source is pyrolyzed bituminous coal which is less expensive than petroleum coke.